Electrical resistor



Aug. 31, 1965 w. J. OSTRANDER ETAL 3,203,330

ELECTRICAL RESISTOR Filed Nov. 24, 1961 2 Sheets-Sheet 1 FIG! U 4 I I lI E z E 5 o. I I l T 3 E 5 IO 5 E E 5 5 2 I E t soo- 2 m 0 E o E 5 U S1&1 'IOOO- E I U E l- D g la! "ISOO- 3 z I Lu I ll 1 1 4O 5O 5O 7O 3O mo30 4O 5O 6O 7O 8O 90 I00 VOLUME PERCENT CHRQMUM VOLUME PERCENT CHROMIUMINVENTORS WILLIAM J. OSTRANDER CARL H. SCHENSKIE ATTORNEY Aug. 31, 1965Filed Nov. 24, 1961 TEMPERATURE COEFFICIENT OF FILM THICKNESS m ANGSTROMUNITS RESISTANCE/RESISTIVITY OHM CM 'c" 30 4O 5O 6O 7O 8O 9O VOLUMEPERCENT CHROMIUM W. J. OSTRANDER ETAL ELECTRICAL RES ISTOR 2Sheets-Sheet 2 IIIIIII IIIIHI I l IIIIIII I I IIHIII I llllllli BULKHRTOMIUM I I I I lllllll FIG.6

I I llllll l I IIIIIII l I lllllll I llllllll I l I I I l SIO SOURCECHROMIUM-SILICON OXIDE MIXTURE FIG.7

C HROMI UM Cr SOURCE I I l l I 456789IOIII2 SAMPL E NUMBER INVENTORSWILLIAM J. 05 TRANDER CARL H. SCHENSKIE ATTORNEY United States Patent3,203,830 ELECTRICAL RESISTOR William J. Ostrander, Harrington, N.J.,and Carl H. Schenskie, Philadelphia, Pa., assignors to InternationalResistance Company, Philadelphia, Pa. 1 Filed Nov. 24, 1961, Ser. No.154,578

4 Claims. (Cl. 117-227) The present invention relates to an electricalresistor and the method of making the same. More particularly, thepresent invention relates to an electrical resistor having a relativelyhigh resistivity and a low temperature coeficient of resistance, and themethod of making the same.

With the development of more complex electronic equipment, it has beenfound necessary to use in such equipment electrical components, such asresistors, which are highly stable. By a highly stable resistor, it ismeant a resistor Whose resistance remains constant or changes onlyslightly when the resistor is in use. It is well known that when aresistor is used in an electrical circuit, the resistor heats up becauseof the electrical current passing therethrough. Also, such resistors aresubjected to changes in temperature because of the environment in whichthe electronic equipment is used. It is also Well known that mostresistance materials change in resistance value when subjected tochanges in temperature. Such a change in resistance value is known asthe temperature coeificient of resistance of the material. Therefore,the temperature coefficient of resistance of a resistor is an importantfactor which atiects the stability of the resistor.

As stated in United States Letters Patent No. 2,847,325 to J. Riseman et211., issued August 12, 1958 entitled Apparatus and Method forEvaporating Films in Certain Types of Electrical Components, resistorshaving a substantially zero temperature coefiicient of resistance can beformed with very thin films of certain metals. Although such resistorsare very stable with regard to temperature, the thin metal films haveonly a low resistivity. Therefore, it is desirable to have a resistorwhich is stable with regard to temperature, but which has a relativelyhigh resistivity of the resistance material.

It is an object of the present invention to provide a novel resistor.

It is another object of the present invention to provide a novelresistor which is stable with regard to temperature.

It is still another object of the present invention to provide aresistor which has a low temperature coeflicient of resistance, and arelatively high resistivity of the resistance material.

It is a further object of the present invention to provide a novelmethod for making a resistor.

It is a still further object of the present invention to provide amethod for making a resistor having a low temperature coefiicient ofresistance and a relatively high resistivity of the resistance material.

Other objects will appear hereinafter.

FIGURE 1 is a sectional view of the resistor of the present invention.

FIGURE 2 is a schematic view of an apparatus for making the resistor ofthe present invention.

FIGURE 3 is a schematic view illustrating the manner of making theresistor of the present invention.

FIGURE 4 is a graphical illustration of the variation in conductivity ofthe resistance material of the present invention with variations in theratio of the components of said material.

FIGURE 5 is a graphical illustration of the variation in the temperaturecoefficient of resisistance of the resist- 3,203,830 Patented Aug. 31,1965 ance material of the present invention with variations in the ratioof the components of said material.

FIGURE 6 is a graphical illustration of the variation in the ratio oftemperature coefficient of resistance to resistivity of the resistancematerial of the present invention with variations in the ratio of thecomponents of said material.

FIGURE 7 is a graphical illustration comparing the thickness of theresistance material film of the present invention to the thickness of ametal film.

For the purpose of illustrating the invention there is shown in thedrawings a form which is presently preferred; it being understood,however, that this invention is not limited to the precise arrangementsand instrumentalities shown.

Referring initially to FIGURE 1, the resistor of the present inventionis generally designated as 10. Resistor 10 comprises a base substrate 12of an electrical insulating material, such as a ceramic or glass, and alayer 14 of the resistance material of the present invention coated onthe surface of the base substrate 12. Caps 16 of an electricallyconductive metal are mounted on the ends of the base substrate 12, andare electrically connected to the resistance material layer 14. Terminalwires 18 of an electrically conductive metal are secured to the caps 16,and project therefrom. Although the base substrate 12 is shown as beingcircular in transverse cross-section, the base substrate may also beilat with the resistance material layer 14 being coated on one or bothsurfaces thereof. Also, the resistor 10 may be provided with any type ofterminals well known in the art in place of the caps 16 and terminalwires 18.

The resistance material layer 14 comprises a homogenous mixture of ametal selected from the group of chromium, molybdenum, tungsten, nickeland alloys of such metals and silicon oxide. As used herein, the termsilicon oxide is meant to refer to the oxide or oxides of silicon in theresistance material layers of the present invention whether it be amixture of the oxides of silicon or only one of the oxides. As will beexplained in detail later, the resistance material layer 14 is formed onthe base substrate 12 by simultaneously evaporating the metal andsilicon monoxide in a vacuum, and depositing the mixture on the surfaceof the base substrate. It has been found that the conductivity, which isthe inverse of the resistivity, and the temperature coetficient ofresistance of the resistance material layer 14 of the present inventionvaries with a variation in the ratio of the metal to the silicon oxidein the resistance material layer. As can be seen in FIGURE 4 in whichthe metal is chromium, as the volume percentage of the chromium in theresistance material layer 14 increases, the conductivity of theresistance material increases very rapidly until the resistance materiallayer 14 contains approximately 50% chromium and 50% silicon oxide.Increasing the volume percentage of chromium in the resistance materiallayer further still increases the conductivity, but at a much slowerrate.

Referring toFIGURE 5, it can be seen that upon increasing the volumepercentage of chromium in the resistance material layer 14, thetemperature coeflicient of resistance of the resistance material changesrapidly from a high negative temperature coeliicient of resistance to azero temperature coefiicient of resistance at approximately 43%chromium. Upon increasing the volume percentage of chromium in theresistance material layer further, the temperature coeiiicient ofresistance of the material increases slightly in a positive direction,and then substantially levels olf.

By comparing FIGURE 4 with FIGURE 5, it can be seen that in the range ofapproximately 42 to 50% chromium the resistance material of the presentinvention has a relatively low temperature coefficient of resistance,and a conductivity which is considerably less than the conductivity ofbulk chromium so that the resistivity of the material is relatively highas compared to the resistivity of the bulk metal. Thus, with aresistance layer 14 of the present invention which contains between 42and 50% chromium by volume, the resistor of the present invention willhave a relatively high resistance and a relatively low temperaturecoefficient of resistance. Furthermore, as can be seen from FIGURE 4, inthe range of 42 to 50% chromium in the resistance material layer 14 ofthe present invention, the resistivity of the resistance material layervaries greatly, approximately tenfold, so that resistors of the presentinvention can be formed over a large range of resistivities and stillhave a relatively low temperature coefiicient of resistance. Thus, theresistance layer 14 of the present invention, which is a homogeneousmixture of chromium and silicon oxide, permits the manufacture ofresistors over a large range of relatively high resistance values withthe temperature coefiicient of resistance of the resistors beingrelatively low so that the resistors are relatively stable with regardto temperature.

Although the resistance material layer 14 of the present invention whichcontains between 42 and 50% by volume of chromium provides a resistorhaving preferred characteristics, it should be understood that theresistance layer of the present invention containing more than 50% orless than 42% by volume of chromium provides resistors having desiredcharacteristics. In the range of less than 42% by volume of chromium,there are provided resistors having very high resistivities and hightemperature coefficients of resistance. Such resistors have applicationwhere there is required a resistor of high resistance and where thetemperature coefficient of resistance is not an important factor, orwhere there is required a resistor having a very large temperaturecoefiicient of resistance, such as a thermistor. In the range of morethan 50% by volume of chromium, there are provided resistors havingtemperature coeflicient of resistance which is substantially uniform andrelatively low over this range and having a resistivity which is higherthan that of bulk chromium.

Another novel feature of the resistance material layer of the presentinvention is shown in FIGURE 6 in which the ratio of the temperaturecoefiicient of resistance to the conductivity is plotted against thecomposition of the resistance material layer. As can be seen, this ratiodedecreases with an increase in the amount of chromium in the resistancematerial layer. As can be seen, this ratio decreases with an increase inthe amount of chromium in the resistance material layer until theresistance material layer of the present invention containsapproximately 60% chromium. The resistance material layer of the presentinvention containing more than 60% chromium has a substantially constantratio of temperature coefficient of resistance to conductivity. Thenovel feature is that this ratio for the resistance material layers ofthe present invention containing more than 60% chromium is approximatelythe same as the ratio of the temperature coefficient of resistance toconductivity for bulk chromium. Thus, the resistance material layers ofthe present invention having more than 60% chromium have the samecharacteristics as bulk chromium with regard to the ratio of thetemperature coeflicient of resistance to conductivity. However, as canbe seen in FIGURES 4 and 5, such resistance material layers of thepresent invention have a relatively low temperature coefiicient ofresistance and a resistivity higher than that of bulk chromium.

Still another feature of the resistance material layer of the presentinvention can be seen in FIGURE 7 in which the thickness of theresistance material layer of the present invention is compared with thethickness of chromium. As can be seen, the resistance material layer ofthe present invention is thicker than a similarly formed layer ofchromium. It is well known that the thicker a resistance material layeris the more stable the layer is electrically with regard not only totemperature but to other conditions, such as oxidation, humidity, etc.Thus, the resistance material layers of the present invention are morestable than pure metal resistance layers of the same resistance.

Although the discussion of the characteristics of the resistancematerial layer of the present invention has been with regard to suchlayers in which the metal is chromium the resistance material layers ofthe present invention in which the metal is either molybdenum, tungsten,nickel or alloys of such metals may have similar characteristics.

As was previously stated, the resistance material layer 14 of thepresent invention is coated on the base substrate 12 by simultaneouslyevaporating the metal and silicon monoxide from separate sources in anevacuated chamber, and condensing the vapors of the metal and thesilicon oxide on the base substrate 12. Although silicon monoxide isevaporated, it is believed that some or all of the silicon monoxidevapors may be oxidized, depending on the particular pressure or the typeof atmosphere in the chamber. Thus, it may be possible to control thetype of silicon oxide achieved in the resistance material layer byeither controlling the pressure in the coating chamber or by leakingcontrolled quantities of certain gases, such as air or oxygen, into thechamber during the coating operation.

Thus, the resistance material layer 14 of the present invention soformed may be either a mixture of the oxides of silicon or merely asingle oxide of silicon. Referring to FIGURE 2, there is schematicallyshown an apparatus for carrying out this method. The apparatus comprisesa base plate 24 and a dome-shaped cover 22, such as a bell jar, mountedon and hermetically sealed to the base plate 20 to provide a chambertherein. An exhaust pump 24 is connected to the chamber within the cover22 by a pipe 26 extending through the base plate Ztl. A pair ofevaporating sources 28 and 30 are mounted in spaced relation in thechamber within the cover 22. The evaporation sources 28 and 30 are of amaterial capable of being heated to the evaporating temperatures of themetal and the silicon monoxide respectively, but which has anevaporating temperature higher than that of the metal and siliconmonoxide. For example, the evaporating source 28 for chromium may be astrip of tantalum having substantially pure chromium coated thereon, andthe evaporating source 3'3 for the silicon monoxide may be a boatshapedpiece of tantalum adapted to contain the silicon monoxide. Theevaporating sources 28 and 30 are electrically connected across sourcesof electrical current, such as the batteries 32 and 34. Although theevaporating sources 28 and 30 are shown as being connected to separatesources of electrical current, they may be connected to the same sourceof electrical current with separate rheostats being provided to controlthe temperature to which each of the evaporating sources is heated. Thebase substrate 12 is suitably supported by any means well known in theart in the chamber within the cover 22, and is positioned over andbetween the evaporating sources 28 and 30.

To coat the surface of the base substrate 12 with the layer 14 of theresistance material of the present invention using chromium as themetal, the chamber within the cover 22 is evacuated to a pressure ofless than 5X 1() millimeters of mercury. The current to the evaporatingsources 28 and 30 is then simultaneously turned on to simultaneouslyheat the evaporating sources to a temperature sufiicient to evaporatethe chromium and the silicon monoxide, which is approximately 1500 C.for chromium and approximately 1100 C. for silicon monoxide. As shown inFIGURE 3, when the evaporating sources 28 and 39 are heated to theevaporating temperatures of the chromium and the silicon monoxide, thevapors of chromium and silicon monoxide diituse in all directions fromthe evaporating sources, as indicated by the lines radiating from theevaporating sources. In the area between the evaporating sources 28 and30, the vapors of the chromium and the silicon monoxide co-mingle. Thus,by placing the base substrate 12 in this area, the mixed vapors of thechromium and the silicon oxide condense on the cooler base substrate soas to provide on the surface of the base substrate the layer 14 of theresistance material of the present invention which is a homogeneousmixture of chromium and silicon oxide.

As is well known, the density of the vapors from an evaporating sourcedecreases with the distance from the source. Thus, from a point directlyover the chromium evaporating source 28 to a point directly over thesilicon monoxide evaporating source 30, the ratio of the amount ofchromium vapors to the amount of silicon monoxide vapors in the mixtureof the vapors varies with the percentage of the chromium vapors in themixture being greater at the point directly over the chromiumevaporating source than at the point directly over the silicon monoxideevaporating source. Therefore, by properly positioning the basesubstrate 12 with regard to the evaporating sources 28 and 30, aresistance material layer 14 of the present invention having any desiredratio of chromium to silicon oxide can be obtained. Thus, knowing fromthe graphs of FIGURES 4 and 5 the percentages of chromium and siliconoxide required in the resistance material layer 14 to obtain a resistorhaving a desired resistivity and temperature coefiicient of resistance,the base substrate 12 can be properly positioned with regard to theevaporating sources 28 and 30. The proper position of the base substrate12 with regard to the evaporating sources 28 and 30 to obtain aresistance material layer 14 having a desired ratio of chromium tosilicon oxide can be obtained either experimentally or theoretically bythe following formula:

where F =volume fraction of chromium W =rate of evaporation of siliconmonoxide W =rate of evaporation of chromium P =density of siliconmonoxide P =density of chromium T ==thickness of film due to siliconmonoxide T =thickness of film due to chromium d=distance between sourcesh distance between plane of substrate and plane of evaporating sourcesy=the distance measured along the substrate plane from a point on thesubstrate to the point directly over one of the evaporating sourcesKnowing the resistivity of the resistance material layer 14, theresistance value per unit area of the base substrate 12 will bedetermined by the thickness of the resistance material layer. Thus, bycontrolling the time that the base substrate 12 is exposed to thechromium and the silicon monoxide vapors a resistor 10 of substantiallyany desired resistance value can be obtained. One well known method ofobtaining a resistor of a desired resistance value is to continuouslymeasure the resistance across the base substrate 12 during the coatingoperation, and stopping the evaporation when the desired resistancevalue is reached.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification as indicating the scope of theinvention.

We claim:

1. An electrical resistor having a relatively high resistance and lowtemperature coefiicient of resistance comprising a base substrate of anelectrical insulating material, and a layer of a resistance material onthe surface of said base substrate, said resistance material layerconsisting essentially of a homogeneous mixture of chromium and siliconoxide with at least 42% by volume of the mixture being chromium.

2. An electrical resistor having a relatively high resistance and lowtemperature coeflicient of resistance comprising a base substrate of anelectrical insulating material, and a layer of a resistance material onthe surface of said base substrate, said resistance material layerconsisting essentially of a homogeneous mixture of chromium and siliconoxide with at least by volume of the mixture being chromium.

3. An electrical resistor having a relatively high resistance and lowtemperature coeflicient of resistance comprising a base substrate of anelectrical insulating material, and a layer of a resistance material onthe surface of said base substrate, said resistance material layerconsisting essentially of between approximately 442 to 50% by volume ofchromium and 58 to 50% by volume of silicon oxide.

4. An electrical resistor having a relatively high resistance and lowtemperature coeflicient of resistance comprising a base substrate of anelectrical insulating material, and a layer of a resistance material onthe surface of said base substrate, said resistance material layerconsisting essentially of approximately 43% by volume of chromium and57% by volume of silicon oxide.

References Cited by the Examiner UNITED STATES PATENTS 2,808,351 10/57Colbert et al. 117-407 X 2,852,416 9/58 McNary et a1. 1l7-227 X FOREIGNPATENTS 834,490 3/52 Germany. 882,174 7/53 Germany.

RICHARD D. NEVIUS, Primary Examiner.

1. AN ELECTRICAL RESISTOR HAVING A RELATIVELY HIGH RESISTANCE AND LOWTEMPERATURE COEFFICIENT TO RESISTANCE COMPRISING A BASE SUBSTRATE OF ANELECTRICAL INSULATING MATERIAL, AND A LAYER OF A RESISTANCE MATERIAL ONTHE SURFACE OF SAID BASE SUBSTRATE, SAID RESISTANCE MATERIAL LAYERCONSISTING ESSENTIALLY OF A HOMOGENEOUS MIXTURE OF CHROMIUM AND SILICONOXIDE WITH AT LEAST 42% BY VOLUME OF THE MIXTURE BEING CHROMIUM.